1. Field of the Invention
The present invention relates to an image forming apparatus provided with a photoconductor, particularly, an amorphous silicon (a-Si) photosensitive drum for charging a surface of the photosensitive drum by contact charging.
2. Description of the Related Art
Heretofore, a device for electrostatically charging a drum-type electrophotographic photoconductor (hereinafter, simply called as “photosensitive drum”) has been composed of a corona charging device designed to expose a surface of a photosensitive drum to corona charge so as to electrostatically charge the surface. In recent years, from the aspect of advantages in lower-level ozone formation and lower power consumption as compared with the corona charging device, a contact charging type image forming apparatus, designed to bring a charging member such as a charging roller in a voltage-applied state into contact with a surface of a photosensitive drum so as to electrostatically charge the surface has come into practical use (see e.g. Japanese Unexamined Patent Publication No. 8-272270).
Further, in place of selenium or OPC (Organic Photo Conductor) conventionally used as a material for a surface layer of a photosensitive drum, an amorphous silicon (a-Si) photoconductor using an amorphous silicon is recently beginning to be used in view of environmental concerns, longer life duration, etc.
The contact charging type image forming apparatus has the following drawbacks. Generally, a conductive rubber roller i.e. a charging roller containing an ion conductive agent is pressingly contacted with a photosensitive drum to realize the contact charging. In pressing the surface of the photosensitive drum by the charging roller, a load exerted to an axially central part of the photosensitive drum is essentially smaller than a load exerted to the axially opposite ends thereof, with the result that a nip width between the photosensitive drum and the charging roller is narrow on the axially central part of the photosensitive drum, as compared with the axially opposite ends thereof. This may lead to a lower charging potential on the axially central part, as compared with the axially opposite ends, which may cause a surface potential distribution non-uniformity of the photosensitive drum, and resultantly cause image formation failure such as fog. The following approach is known to eliminate the drawback. Specifically, a bias voltage generated by superimposing an AC voltage to a DC voltage is applied to the charging roller to suppress the surface potential distribution non-uniformity due to the nip distance difference.
In the case where an amorphous silicon photoconductor is used, a nip distance difference between the charging roller and the photosensitive drum greatly affects the surface potential distribution of the photosensitive drum. This is because the amorphous silicon photoconductor has a lower chargeability as compared with an organic photoconductor such as OPC. Accordingly, the surface potential distribution non-uniformity cannot be sufficiently eliminated even by the approach of superimposing the AC bias voltage in the image forming apparatus provided with the amorphous silicon photosensitive drum.
Further, there is a case that a surface potential of a photosensitive drum may be fluctuated due to a charge removing light amount distribution non-uniformity. A generally used charge removing device has, as a light source, an LED array having LEDs (light emitting diodes) in the order of about 15 to 40, which are arrayed in a direction of a rotational axis of a photosensitive drum for emitting charge removing light to project the charge removing light toward the surface of the photosensitive drum axially linearly. If the charge removing light emitted from the LED array is projected axially uniformly onto the surface of the photosensitive drum, the chargeability of the photosensitive drum is made uniform. If, however, the amount of the charge removing light is fluctuated axially, a charging fluctuation may occur, which may cause a surface potential distribution non-uniformity of the photosensitive drum, and resultantly lead to image formation failure such as fog.